1. Field of the Invention
The present invention concerns a method of assigning a spreading sequence to a user of a telecommunications network, such as a Multi-Carrier Code Division Multiple Access transmission network, better known by the name MC-CDMA network.
2. Discussion of the Relevant Art
Among the new communication systems making it possible to manage, simultaneously and in one and the same frequency band, a number of communications between distinct users, the Multi-Carrier Code Division Multiple Access transmission technique, better known by the name MC-CDMA technique, is known. This transmission technique combines both the principles of Orthogonal Frequency Division Multiplex transmission known by the name OFDM and those of the Code Division Multiple Access (CDMA) technique.
FIG. 1 depicts a block diagram illustrating an example of the operation of a transmitter using the MC-CDMA transmission technique. This example represents an outgoing route transmission, that is to say from a base station to a mobile station. The n-th data item of the k-th user d(k)[n] is fed to multipliers 10 to 1N−1 respectively provided for multiplying it by the elements cm(k) (m being between 0 and N−1) of a sequence referred to as a spreading sequence, and then to modulators 20 to 2N−1 for respectively modulating it on sub-carriers at the frequency (fc+m.F/Tb) where fc is the original frequency of the transmitted signal and F/Tb is the spacing between two consecutive sub-carriers, F being an integer and Tb being the duration of the data item d(k)[n], excluding a guard interval. All the sub-carriers are added up in an adder 30 to form the transmitted signal s(k)(n,t) which can therefore be expressed in the form:             s              (        k        )              ⁡          (              n        ,        t            )        =                              d                      (            k            )                          ⁡                  [          n          ]                    ⁢                        ∑                      m            =            0                                N            -            1                          ⁢                              c            m                          (              k              )                                ⁢                      cos            ⁡                          [                              2                ⁢                                  π                  ⁡                                      (                                                                  f                        c                                            +                                              m                        ⁢                                                  F                                                      T                            b                                                                                                                )                                                  ⁢                t                            ]                                ⁢                                          ⁢          if          ⁢                                          ⁢          t                      ⋐          [              0        ,                  T          b                    ]                      s(k)(n,t)=0 otherwise        
It should be noted that, in this particular example, the assembly of the modulators 20 to 2N−1 and of the adder 30 can be implemented by an inverse Fourier transform.
It should be noted that, for reasons of clarity, it has been considered that the length N of each spreading sequence is equal to the number M of modulation sub-carriers, which is not necessarily the case in all MC-CDMA systems.
The assumption has also been made, for reasons of clarity, that a single spreading sequence is assigned per user, which is not necessarily the case.
It is known that the propagation channel can be obstructed by houses and other obstacles situated between the transmitter and the receiver. The transmitted signal is then propagated on multiple paths, each path being delayed and attenuated differently. It should be understood that the propagation channel then acts as a filter whose transfer function h(f,t) varies with time.
The contribution denoted sm to the transmitted signal s(n,t) of each carrier m of data items d intended for K users can be expressed as follows:       s    m    =            ∑              k        =        1            K        ⁢                  d                  (          k          )                    ⁢              c        m                  (          k          )                    
In view of the complex attenuation denoted hm(p) induced by the transmission channel at the receiver of the user of rank p, the signal received, in the synchronous case and on the outgoing route, on each sub-carrier of rank m can then be expressed as follows:       r    m          (      p      )        =                    h        m                  (          p          )                    ⁢                        ∑                      k            =            1                    K                ⁢                  (                                    d                              (                k                )                                      ⁢                          c              m                              (                k                )                                              )                      +          n      m              (        p        )                            where nm(p) represents the sample of additive white Gaussian noise on the carrier of rank m.        
The ability of MC-CDMA transmission systems to provide orthogonality between the signals of the different users in the network (and therefore to prevent any interference between these signals) depends on the intercorrelation properties of the spreading sequences which are assigned to the users of the network.
Typically, in the case of transmissions on a mobile radio channel from a base station to a set of mobile stations, the signals intended for each user are transmitted synchronously. Under these conditions, Walsh-Hadamard spreading sequences can be used to guarantee orthogonality between the users if the channel is not frequency selective.
In the known MC-CDMA systems, the assigning of spreading sequences does not, for one and the same family of spreading sequences (Walsh-Hadamard sequences of length N, Gold sequences, etc.), obey precise rules in order that the interference related to the frequency selectivity of the channel is minimized.
However, in actual fact, the present invention is based on the idea that the signal which is received by a receiver of an MC-CDMA system has a component which is related to the interference with the other users, interference which, contrary to what is commonly accepted, depends on the sequences assigned to these users in the same family of sequences used by the transmission system.
This is because, after unspreading, the signal v(p) received by the user p can be expressed in the form:                               v                      (            p            )                          =                              ∑                          m              =              1                        N                    ⁢                                    c              m                              (                p                )                                      ⁡                          [                                                                    h                    m                                          (                      p                      )                                                        ⁢                                                            ∑                                              k                        =                        1                                            K                                        ⁢                                          (                                                                        d                                                      (                            k                            )                                                                          ⁢                                                  c                          m                                                      (                            k                            )                                                                                              )                                                                      +                                  n                  m                                      (                    p                    )                                                              ]                                                              =                              ∑                          m              =              1                        N                    ⁢                      (                                                            h                  m                                      (                    p                    )                                                  ⁢                                                      ∑                                          k                      =                      1                                        K                                    ⁢                                      (                                                                  c                        m                                                  (                          p                          )                                                                    ⁢                                              c                        m                                                  (                          k                          )                                                                    ⁢                                              d                                                  (                          k                          )                                                                                      )                                                              +                              z                m                                  (                  p                  )                                                      )                                                  =                                            d                              (                p                )                                      ⁢                                          ∑                                  m                  =                  1                                N                            ⁢                              h                m                                  (                  p                  )                                                              +                                    ∑                                                k                  =                  1                                ,                                  k                  ≠                  p                                            K                        ⁢                                          d                                  (                  k                  )                                            ⁢                                                ∑                                      m                    =                    1                                    N                                ⁢                                  (                                                            h                      m                                              (                        p                        )                                                              ⁢                                          c                      m                                              (                        p                        )                                                              ⁢                                          c                      m                                              (                        k                        )                                                                              )                                                              +                                    ∑                              m                =                1                            N                        ⁢                          z              m                              (                p                )                                                        
It is assumed here that |cm(p)|2=1 and the notation zm(p)=nm(p)×cm(p) is used.
It should be noted that three contributions are thus revealed in the expression of the signal received by the user p: the desired signal (first term), interference associated with the presence of other users (second term) and noise (third term).
The above relationship can also be written in the form:       v          (      p      )        =                    d                  (          p          )                    ⁢                        ∑                      m            =            1                    N                ⁢                  h          m                      (            p            )                                +                  ∑                              k            =            1                    ,                      k            ≠            p                          K            ⁢                        d                      (            k            )                          ⁢                  I          ⁡                      (                          h              ,              p              ,              k                        )                                +                  ∑                  m          =          1                N            ⁢              z        m                  (          p          )                                    where the term I(h,p,k) is an interference term representing all the interference induced between the two sequences of index p and k, taking into account the frequency selectivity of the channel at the receiver of the user of the sequence of index p and which is therefore equal to       ∑          m      =      1        N    ⁢            h      m              (        p        )              ⁢          c      m              (        p        )              ⁢                  c        m                  (          k          )                    .              
In order to correct the effect of this interference with the other users, implementation is known, at the receiver, of an equalization step whose coefficient of equalization gm takes a complex value which affects each carrier of rank m so that the apparent transfer function h′m(p) of the transmission channel seen by the receiver can be written in the form:h′m(p)=hm(p)×gm 
The modified interference term I′ (h, p, k) resulting from the fact of the distortion brought about by the transmission channel is now written:             I      ′        ⁡          (              h        ,        p        ,        k            )        =            ∑              m        =        1            N        ⁢                  h        m                  ′          ⁡                      (            p            )                              ⁢              c        m                  (          p          )                    ⁢              c        m                  (          k          )                    
A first simplistic approach to the equalization consists of making the apparent transfer function h′m(p) equal to 1 in order to completely restore orthogonality. However, this approach is not used since it increases the noise too much, which degrades the performance of the transmission system. In practice, the equalization provides a compromise between the restoration of orthogonality, that is to say the reduction of inter-user interference, and minimization of the effects of the noise. The interference is therefore never totally removed.
The aim of the invention is therefore to propose a method which makes it possible to attenuate the effects of the interference term (the second in the preceding equation) on the performance of the transmission system under consideration.
To that end, the present invention concerns a method of assigning one or more spreading sequences to a user of a Multi-Carrier Code Division Multiple Access transmission network. This method is characterised in that it consists of assigning, to the said user, the said spreading sequence or the said spreading sequences, taking into account a predetermined set of spreading sequences from among a set of possible sequences.